Hard coating for cutting tool

ABSTRACT

A hard coating includes a thin film layer which has a total thickness of 0.5-10 μm and has an overall composition of Al 1-a-b Ti a Me b N (0.2&lt;a≤0.6, 0&lt;b≤0.15), where Me is a nitride constituent element having a thermal expansion coefficient of greater than 2.7×10 −6 /° C. and less than 9.35×10 −6 /° C., wherein the thin film layer has a structure in which a nano-multilayered-structure of thin layers A, B and C, thin layer B being disposed between thin layer A and thin layer C, is repeatedly laminated at least once.

TECHNICAL FIELD

The present invention relates to a hard coating for cutting tools, and more particularly, to a hard coating which includes an AlTiMeN layer constituting a nano-multilayer structure, may mitigate the occurrence of a thermal crack even though a phase decomposition occurs during a cutting process and may thereby be used suitably for medium-to-low speed interrupted cutting.

BACKGROUND ART

In order to develop a high hardness cutting tool material, various multilayer film systems based on TiN have been proposed since the late 1980's.

For example, when a multi-layered film is formed by alternately and repeatedly laminating TiN or VN with a thickness of several nanometers, despite the differences in the lattice constants of single layers, a matching interface is formed between the layers to form a so-called superlattice having one lattice constant, and thus, a high hardness of at least two times the general hardness of each single film may be achieved. A variety of attempts have been made to apply this phenomenon to a thin film for a cutting tool.

Reinforcing mechanisms used for such superlattice coating include the Koehler's model, the Hall-Petch relationship, the coherency strain model, and the like, and these reinforcing mechanisms increase hardness by controlling the differences in lattice constants and elastic moduli of materials A and B and a lamination period when the materials A and B are alternately deposited.

Recently, for example, as disclosed in Patent document 1 (Korean Patent Publication No. 2013-0123238), hard coatings for a cutting tool provided with various nano-multilayer structures, in which nitrides with various compositions such as AlTiN, TiAlN, AlTiMeN (where, Me is a metal element) are alternately laminated to achieve a remarkably improved physical property compared to a single film, have been proposed.

However, nitride thin films such as AlTiN, TiAlN, or AlTiMeN have limitations in that a phase decomposition into AlN, TiN, or MeN occurs due to a high temperature and a high pressure during a cutting process, and since the difference in thermal expansion coefficients of phase-decomposed AlN, TiN, or the like is excessively large, a thermal crack easily occurs in an initial stage during a cutting process such as medium-to-low speed interrupted milling processing and thereby reduces the tool life.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention is provided to solve the abovementioned limitations of conventional arts and the purpose of the present invention is to provide a hard coating for cutting tools, which includes a nitride layer such as AlTiN, TiAlN, or AlTiMeN and solves the limitation of the reduction of a tool life due to a thermal crack.

Technical Solution

As a means for solving the above mentioned limitation, the present invention provides a hard coating formed on the surface of a base material for a cutting tool by a PVD method, and the hard coating is characterized by including a thin film layer which has a total thickness of 0.5-10 μm and has an overall composition of Al_(1-a-b)Ti_(a)Me_(b)N (0.2<a≤0.6, 0<b≤0.15), where Me is a nitride constituent element having a thermal expansion coefficient of greater than 2.7×10⁻⁶/° C. and less than 9.35×10⁻⁶/° C., wherein the thin film layer has a structure in which a nano-multilayered-structure of thin layers A, B and C, thin layer B being disposed between thin layer A and thin layer C, is repeatedly laminated at least once, the thin film layer satisfies the relationship of kA>kB>kC, where kA is the thermal expansion coefficient of thin layer A, kB is the thermal expansion coefficient of thin layer B, and kC is the thermal expansion coefficient of thin layer C, thin layer A has a composition of Ti_(1-a)Al_(a)N (0.3≤a<0.7), thin layer B has a composition of Ti_(1-y-z)Al_(y)Me_(z)N (0.3≤y<0.7, 0.01≤z<0.5), and thin layer C has a composition of Al_(1-x)Ti_(x)N (0.3≤x≤0.7).

Here, the thermal expansion coefficients of thin layer A, thin layer B, and thin layer C may be values of respective inherent thermal expansion coefficients of single element nitrides times respective composition ratios.

In addition, Me may include one or more selected from among Si and Groups 4a, 5a, and 6a elements.

In addition, Me may include one or more selected from among Si, Zr, Hf, V, Ta, and Cr.

In addition, the difference in the thermal expansion coefficients of Me nitrides and the thermal expansion coefficients of the AlN and TiN may be at least 1.0×10⁻⁶/° C.

Advantageous Effects

An AlTiMeN nitride layer included in a hard coating according to the present invention is formed in a multilayer nanostructure, and a thin layer A, a thin layer B, and a thin layer C which constitute the multilayer nanostructure is configured such that thin layer B is disposed between thin layer A and thin layer C, and the thermal expansion coefficients of thin layer B satisfies the relationship of kA>kB>KC between the thermal expansion coefficients of thin layers. Thus, even though a phase decomposition occurs during a cutting process, the occurrence of thermal cracks may be remarkably reduced, and thus, the tool life may be remarkably prolonged during a cutting process such as medium-to-low speed interrupted milling processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is view for illustrating a nanostructure according to an embodiment of the present disclosure.

FIG. 2 is view for illustrating a nanostructure according to another embodiment of the present disclosure.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter the present invention will be described in more detail on the basis of exemplary preferred example of the present invention, but the present invention is not limited to the examples below.

To solve the limitation of a thermal cracks due to a phase decomposition during a cutting process in a hard coating for cutting tools which includes layers of TiAlN, AlTiN, AlTiMeN, and the like, the present inventors has been carried out intensive research. As a result, the present inventors find that as illustrated in FIG. 1, a thin layer having an overall composition of Al_(1-a-b)Ti_(a)Me_(b)N (0.2<a≤0.6, 0<b≤0.15) is formed from a nano-multilayer structure which is formed by alternately and repeatedly laminating a structure in which an AlTiMeN layer is disposed between an AlTiN thin layer and a TiAlN thin layer, and when a nitride having a thermal expansion coefficient between those of the AlTiMeN and the AlTiN is used for the AlEiMeN layer, the occurrence of thermal cracks due to phase decomposition is remarkably reduced during a cutting process and the tool life may be prolonged. Thus, the present inventors arrive at the present invention.

A hard coating according to the present invention is formed on the surface of a base material for a cutting tool by a PVD method, and the hard coating is characterized by including a thin film layer which has a total thickness of 0.5⁻¹⁰ μm and has an overall composition of Al_(1-a-b)Ti_(a)Me_(b)N (0.2<a≤0.6, 0<b≤0.15), where Me is a nitride constituent element having a thermal expansion coefficient of greater than 2.7×10⁻⁶/° C. and less than 9.35×10⁻⁶/° C., wherein the thin film layer has a structure in which a nano-multilayered-structure of thin layers A, B and C, thin layer B being disposed between thin layer A and thin layer C, is repeatedly laminated at least once, the thin film layer satisfies the relationship of kA>kB>kC, where kA is the thermal expansion coefficient of thin layer A, kB is the thermal expansion coefficient of thin layer B, and kC is the thermal expansion coefficient of thin layer C, thin layer A has a composition of Ti_(1-a)Al_(a)N (0.3≤a<0.7), thin layer B has a composition of Ti_(1-y-z)Al_(y)Me_(z)N (0.3≤y<0.7, 0.01≤z<0.5), and thin layer C has a composition of Al_(1-x)Ti_(x)N (0.3≤x<0.7).

When the thickness of the hard coating according to the present invention is 0.5 μm or less, it is difficult to exhibit a characteristic inherent in a thin film, and when the thickness is greater than 10 μm, the thickness is favorably 0.5-10 μm, considering the fact that compressive stress accumulated in the thin film is proportional to the thickness of the thin film and a time period due to a manufacturing characteristics of the thin film formed through the PVD method.

In addition, the Me content (b) is favorably 0.15 or less because it is difficult to mitigate the difference in thermal expansion coefficient between AlTiN and TiAlN when Me is not added, and when the Me content (b) is greater than 0.15, overall wear resistance of the thin film is degraded as the hardness of MeN itself is lower than that of TiN among a phase-decomposed nitrides due to a high temperature generated during a cutting process.

In thin layer A, the Al content (a) is favorably in a range of 0.3-0.7 because when the Al content (a) is less than 0.3, Al which has a smaller atomic radius than Ti is replaced and the soluble amount of Al is reduced, hardness and wear resistance of the thin film are thereby degraded, the formation of TiO₂ oxide becomes easy in a high-temperature atmosphere during a cutting process and Ti elements inside the thin film is diffused to the outside, and thus, a high-temperature hardness may be degraded due to exhaustion of Ti elements, and when the Al content (a) is greater than 0.7, brittleness increases due to the formation of a phase of hexagonal B4 structure, wear resistance is thereby degraded, and the tool life may be shortened.

In thin layer B, the Al content (y) is favorably in a range of 0.3-0.7 because when the Al content (a) is less than 0.3, Al which has a smaller atomic radius than Ti is replaced and the soluble amount of Al is reduced, hardness and wear resistance of the thin film are thereby degraded, the formation of TiO₂ oxide becomes easy in a high-temperature atmosphere during a cutting process and Ti elements inside the thin film is diffused to the outside, and thus, a high-temperature hardness may be degraded due to exhaustion of Ti elements, and when the Al content (a) is greater than 0.7, brittleness increases due to the formation of a phase of hexagonal B4 structure, wear resistance is thereby degraded, and the tool life may be shortened.

In addition, the Me content (z) is favorably 0.01-0.5 because: when the Me content (z) is less than 0.01, a grain refining effect and a hardness increasing effect are unsatisfactory due to infiltration or substitutional solution reinforcing effect, and a merit of mitigating the difference in thermal expansion coefficient between phase-decomposed AlN and TIN is unsatisfactory because the content of phase-decomposed MeN nitrides due to a high temperature generated during a cutting process; and when the Me content (z) is greater than 0.5, as the content of MeN, which has a lower self-hardness than TiN, among the phase-decomposed nitrides by a high temperature is increased, there is a tendency of decrease in wear resistance.

Table 1 below shows thermal expansion coefficients of nitrides for each metal element.

TABLE 1 Thermal expansion coefficient (×10⁻⁶/ Classification ° C.) Hardness (GPa) AlN 2.7 12 TiN 9.35 23 ZrN 7.24 27 HfN 6.9 16.3 VN 8.7 14.2 NbN 10.1 13.3 TaN 8 21 CrN 3.5 22 Si₃N₄ 3.2 17

The values of the thermal expansion coefficients in Table 1 above are extracted from “handbook of refractory carbide and nitrides (by Hugh O, Pierson)”.

In addition, Me is favorably a nitride-forming element such that when the Me is added to AlTiN, the thermal expansion coefficients thereof favorably fall between 2.7×10⁻⁶/° C. and 9.35×10⁻⁶/° C. which are the thermal expansion coefficients of AlN and TiN as shown in Table 1 above.

In addition, the Me is favorably a nitride-forming element which has a difference in thermal expansion coefficient of at least 1.0×10⁻⁶/° C. with the thermal expansion coefficient of AlTiN or TiAlN. However, since micro-structure of the nitrides formed when the Me is added is refined and the physical properties of a thin film may e improved, it is desirable that the Me be selected considering the thermal expansion coefficient when the Me is added and the degree of improving thin film characteristics due to addition of the Me.

For example, Me may include one or more selected from among Si and Groups 4a, 5a, and 6a elements. More favorably, Me may include one or more selected from among Si, Zr, Hf, V, Ta, and Cr.

Example

In the present invention, when an alternate repetitive lamination was performed, as shown in FIGS. 1 and 2, in which Si, Zr, Hf, V, Ta, Cr or Si was added, or V and Si were simultaneously added as the Me element included in an AlTiMeN thin film, a hard coating having a laminated nanostructure in which thin layer C was located between thin layer A and thin layer B, and for the comparison with these examples, hard coatings having a lamination structure such as the tables below according to respective comparative examples were prepared.

At this point, the substrate on which a hard coating was formed was formed by using a WC-10 wt % Co cemented carbide with a model number of APMT1604PDSR-MM.

In addition, each of nano-multilayers constituting the hard coating was formed through an arc ion plating method, which was the physical vapor deposition (PVD), so that a hard coating having cross-sectional structures illustrated in FIGS. 1 and 2 was formed.

Specifically, in examples of the present invention, arc ion plating was performed on a cemented carbide substrate formed of WC-10 wt % Co by using AlTi, TiAl, and AlTiMe arc target, and at this point, the initial vacuum pressure was 8.5×10⁻⁵ Torr or less, and N2 was injected as a reaction gas. In addition, a method, in which the reaction gas pressure was set to 50 mTorr or less, the coating temperature was set to 400-500° C., and the substrate bias voltage of −20V to −150V was applied during coating, was used. The coating conditions may be set different from the examples of the present invention according to the characteristics and conditions of equipment used.

A multilayered thin film structure, in which hard coatings of laminated nanostructures were sequentially laminated in the order of TiAlN—AlTiMeN—AlTiN (an example), TiAlN—AlTiMeN—AlTiN—AlTiMeN (an example), TiAlN—AlTiN (a comparative example), TiAlN—AlTiN—TiAlN—AlTiMeN (a comparative example) through the above-mentioned coating method, was applied.

A hard coating according to an example of the present invention was completed by laminating each unit layer of the above nano-multilayers was laminated 13-20 times in a thickness of 15-20 nm such that the thickness of the hard coating film fell within a range of 2.7-3.4 μm.

Meanwhile, if necessary, thin-films with various shapes may of course be formed on the hard coating formed according to the example of the present invention. In addition, the hard coating according to the example of the present invention uses the physical vapor deposition method (PVD), and may have the maximum thin-film thickness of approximately 10 μm.

TABLE 2 Zr-containing A-B-C three-layer repetitive lamination structures Me:Zr Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Zr N (×10⁻⁶/° C.) Example 1-1 A 50 50 0 6.025 B 50 40 10 5.814 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 29.6 23.6 1.8 45 value Example 1-2 A 50 50 0 6.025 B 50 25 25 5.4975 C 67 33 0 4.8945 Theoretical 55.7 41 8.3 composition ratio Analyzed 32.6 20.6 4.3 42.5 value Example 1-3 A 50 50 0 6.025 B 50 10 40 5.181 C 67 33 0 4.8945 Theoretical 55.7 31 13.3 composition ratio Analyzed 30.6 18.1 7.3 44 value Comparative A 50 50 0 6.025 example 1-6 B 50 0 50 4.97 C 67 33 0 4.8945 Theoretical 55.7 27.7 16.7 composition ratio Analyzed 30.7 15.6 9.5 44.2 value

TABLE 3 Zr-containing A-B-C-B four-layer repetitive lamination structures Me:Zr Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Zr N (×10⁻⁶/° C.) Example 1-4 A 50 50 0 6.025 B 50 40 10 5.814 C 67 33 0 4.8945 B 50 40 10 5.814 Theoretical 54.25 40.75 5 composition ratio Analyzed 29.8 22.6 2.8 44.8 value Example 1-5 A 50 50 0 6.025 B 50 25 25 5.814 C 67 33 0 4.8945 B 50 25 25 5.4975 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 30.2 18.8 6.9 44.1 value Comparative A 50 50 0 6.025 example 1-7 B 50 40 10 5.181 C 67 33 0 4.8945 B 50 10 40 5.181 Theoretical 54.25 25.75 20 composition ratio Analyzed 28.8 15.2 11 45 value Comparative A 50 50 0 6.025 example 1-8 B 50 0 50 4.97 C 67 33 0 4.8945 B 50 0 50 4.97 Theoretical 54.25 20.75 25 composition ratio Analyzed 29.8 12.4 13.7 44.1 value

TABLE 4 Zr-containing other lamination structures Me:Zr Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Zr N (×10⁻⁶/° C.) Comparative A 50 50 0 6.025 Example 1-1 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 40 10 5.814 Theoretical 54.25 43.25 2.5 composition ratio Comparative Analyzed 30.8 24.8 1.4 43 Example 1-2 value A 50 50 0 6.025 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 25 25 5.4975 Theoretical 54.25 39.5 6.25 composition ratio Comparative Analyzed 29.6 21.8 3.6 45 Example 1-3 value A 50 50 0 6.025 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 10 40 5.181 Theoretical 54.25 35.75 10 composition ratio Comparative Analyzed 30.1 20.2 6.5 43.2 example 1-4 value A 50 50 0 6.025 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 0 50 4.97 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 29.8 18.3 7.8 44.1 value

TABLE 5 Comparative example containing no AlTiMeN Me:none Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Zr N (×10⁻⁶/° C.) Comparative A 50 50 0 6.025 example 5 C 67 33 0 4.8945 (common) Theoretical 58.5 41.5 0 composition ratio Analyzed 32.7 24.8 0 42.5 value

TABLE 6 Hf-containing A-B-C three-layer repetitive lamination structures Me:Hf Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Hf N (×10⁻⁶/° C.) Example 2-1 A 50 50 0 6.025 B 50 40 10 5.78 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 32.6 22.8 2.3 42.3 value Example 2-2 A 50 50 0 6.025 B 50 25 25 5.78 C 67 33 0 4.8945 Theoretical 55.7 36 8.3 composition ratio Analyzed 30.6 19.8 4.8 44.8 value Example 2-3 A 50 50 0 6.025 B 50 10 40 5.78 C 67 33 0 4.8945 Theoretical 55.7 31 13.3 composition ratio Analyzed 30.6 18.1 8 43.3 value Comparative A 50 50 0 6.025 Example 2-6 B 50 0 50 5.78 C 67 33 0 4.8945 Theoretical 55.7 27.7 16.7 composition ratio Analyzed 31 15.7 9.2 44.1 value

TABLE 7 Hf-containing A-B-C-B four-layer repetitive lamination structures Me:Hf Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Hf N (×10⁻⁶/° C.) Example 2-4 A 50 50 0 6.025 B 50 40 10 5.78 C 67 33 0 4.8945 B 50 40 10 5.78 Theoretical 54.25 40.75 5 composition ratio Analyzed 30.5 22.6 2.8 44.1 value Example 2-5 A 50 50 0 6.025 B 50 25 25 5.78 C 67 33 0 4.8945 B 50 25 25 5.78 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 28.8 19.3 6.9 45 value Comparative A 50 50 0 6.025 example 2-7 B 50 10 40 5.78 C 67 33 0 4.8945 B 50 10 40 5.78 Theoretical 54.25 25.75 20 composition ratio Analyzed 29.8 14.2 11.1 44.9 value Comparative A 50 50 0 6.025 example 2-8 B 50 0 50 5.78 C 67 33 0 4.8945 B 50 0 50 5.78 Theoretical 54.25 20.75 25 composition ratio Analyzed 28.8 12.4 13.8 45 value

TABLE 8 Hf-containing other lamination structures Me:Hf Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Hf N (×10⁻⁶/° C.) Comparative A 50 50 0 6.025 example 2-1 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 40 10 5.78 Theoretical 54.25 39.5 6.25 composition ratio Analyzed 30 22.1 5.4 42.5 value Comparative A 50 50 0 6.025 Example 2-2 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 25 25 5.4125 Theoretical 54.25 39.5 6.25 composition ratio Analyzed 30 22.1 5.4 42.5 value Comparative A 50 50 0 6.025 example 2-3 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 10 40 5.045 Theoretical 54.25 35.75 10 composition ratio Analyzed 29.8 19.5 5.7 45 value Comparative A 50 50 0 6.025 example 2-4 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 0 50 4.8 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 29.6 18.3 6.9 45.2 value

TABLE 9 V-containing A-B-C three-layer repetitive lamination structures Me:V Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti V N (×10⁻⁶/° C.) Example 3-1 A 50 50 0 6.025 B 50 40 10 5.96 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 30.5 23.6 1.9 44 value Example 3-2 A 50 50 0 6.025 B 50 25 25 5.8625 C 67 33 0 4.8945 Theoretical 55.7 36 8.3 composition ratio Analyzed 30.6 19.8 4.8 44.8 value Comparative A 50 50 0 6.025 example 3-7 B 50 0 50 5.7 C 67 33 0 4.8945 Theoretical 55.7 27.7 16.7 composition ratio Analyzed 30.6 16.4 9.2 43.8 value Comparative A 50 50 0 6.025 example 3-8 B 10 50 40 8.425 C 67 33 0 4.8945 Theoretical 42.3 44.3 13.3 composition ratio Analyzed 23.3 25.4 7.3 44 value

TABLE 10 V-containing A-B-C-B four-layer repetitive lamination structures Me:V Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti V N (×10⁻⁶/° C.) Example 3-4 A 50 50 0 6.025 B 50 40 10 5.96 C 67 33 0 4.8945 B 50 40 10 5.96 Theoretical 54.25 40.75 5 composition ratio Analyzed 29.8 22.3 2.9 44.9 value Example 3-5 A 50 50 0 6.025 B 50 25 25 5.8625 C 67 33 0 4.8945 B 50 25 25 5.8625 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 30 18.5 6.9 44.6 value Comparative A 50 50 0 6.025 example B 50 10 40 5.8625 3-11 C 67 33 0 4.8945 B 50 10 40 5.8625 Theoretical 54.25 25.75 20 composition ratio Analyzed 29.8 14.2 12.3 43.7 value Comparative A 50 50 0 6.025 example B 50 0 50 5.7 3-12 C 67 33 0 4.8945 B 50 0 50 5.7 Theoretical 54.25 20.75 25 composition ratio Analyzed 28.1 13.2 13.8 44.9 value Comparative A 50 50 0 6.025 example B 10 50 40 8.425 3-13 C 67 33 0 4.8945 B 10 50 40 8.425 Theoretical 34.25 45.75 20 composition ratio Analyzed 19 25.2 11 44.8 value

TABLE 11 V-containing other lamination structures Me:V Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti V N (×10⁻⁶/° C.) Comparative A 50 50 0 6.025 example 3-1 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 40 10 5.96 Theoretical 54.25 43.25 2.5 composition ratio Analyzed 29.8 23.8 1.4 45 value Comparative A 50 50 0 6.025 Example 3-2 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 25 25 5.8625 Theoretical 54.25 39.5 6.25 composition ratio Analyzed 29.9 21.7 3.4 44.9 value Comparative A 50 50 0 6.025 example 3-3 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 10 40 5.765 Theoretical 54.25 35.75 10 composition ratio Analyzed 30.3 20.7 6.5 42.5 value Comparative A 50 50 0 6.025 example 3-4 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 0 50 5.7 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 26.8 21.3 8.2 43.7 value Comparative 50 50 0 6.025 example 3-5 67 33 0 4.8945 50 50 0 6.025 10 50 40 8.425 44.25 45.75 10 24.3 25.2 6 44.5

TABLE 12 Nb-containing A-B-C three-layer repetitive lamination structures Me:Nb Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Nb N (×10⁻⁶/° C.) Comparative A 50 50 0 6.025 Example 4-1 B 50 40 10 6.1 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 31.1 22.6 1.8 44.5 value Comparative A 50 50 0 6.025 Example 4-2 B 50 25 25 6.2125 C 67 33 0 4.8945 Theoretical 55.7 36 8.3 composition ratio Analyzed 30.6 19.8 4.7 44.9 value Comparative A 50 50 0 6.025 Example 4-3 B 50 10 40 6.325 C 67 33 0 4.8945 Theoretical 55.7 31 13.3 composition ratio Analyzed 30.6 17.1 7.6 44.7 value Comparative A 50 50 0 6.025 example 4-4 B 50 0 50 6.4 C 67 33 0 4.8945 Theoretical 55.7 27.7 16.7 composition ratio Analyzed 30.1 15.2 9.2 45.5 value

TABLE 13 Nb-containing A-B-C-B four-layer repetitive lamination structures Me:Nb Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Nb N (×10⁻⁶/° C.) Comparative A 50 50 0 6.025 example B 50 40 10 6.1 4-5 C 67 33 0 4.8945 B 50 40 10 6.1 Theoretical 54.25 40.75 5 composition ratio Analyzed 29.8 22.4 2.8 45 value Comparative A 50 50 0 6.025 example B 50 25 25 6.2125 4-6 C 67 33 0 4.8945 B 50 25 25 6.2125 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 29.8 20.3 6.9 42.5 value Comparative A 50 50 0 6.025 example B 50 10 40 6.325 4-7 C 67 33 0 4.8945 B 50 10 40 6.325 Theoretical 54.25 25.75 20 composition ratio Analyzed 29.8 15.2 12.5 41.5 value Comparative A 50 50 0 6.025 example B 50 0 50 6.4 4-8 C 67 33 0 4.8945 B 50 0 50 6.4 Theoretical 54.25 20.75 25 composition ratio Analyzed 29.8 11.4 13.8 45 value

TABLE 14 Ta-containing A-B-C three-layer repetitive lamination structures Me:Ta Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Ta N (×10⁻⁶/° C.) Example 5-1 A 50 50 0 6.025 B 50 40 10 5.89 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 30.4 22.6 1.8 45.2 value Example 5-2 A 50 50 0 6.025 B 50 25 25 5.6875 C 67 33 0 4.8945 Theoretical 55.7 36 8.3 composition ratio Analyzed 30.6 19.8 4.9 44.7 value Example 5-3 A 50 50 0 6.025 B 50 10 40 5.485 C 67 33 0 4.8945 Theoretical 55.7 31 13.3 composition ratio Analyzed 30.4 17.1 7.5 45 value Comparative A 50 50 0 6.025 example 5-6 B 50 0 50 5.35 C 67 33 0 4.8945 Theoretical 55.7 27.7 16.7 composition ratio Analyzed 31.6 15.2 10.2 43 value

TABLE 15 Ta-containing A-B-C-B four-layer repetitive lamination structures Me:Ta Thermal expansion coefficient Lamination Composition (at %) (×10⁻⁶/ Division structure Al Ti Ta N ° C.) example 5-4 A 50 50 0 6.025 B 50 40 10 5.89 C 67 33 0 4.8945 B 50 40 10 5.89 Theoretical 54.25 40.75 5 composition ratio Analyzed 29.7 22.4 2.8 45.1 value example 5-5 A 50 50 0 6.025 B 50 25 25 5.6875 C 67 33 0 4.8945 B 50 25 25 5.6875 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 29.8 18.6 7 44.6 value Comparative A 50 50 0 6.025 example 5-7 B 50 10 40 5.485 C 67 33 0 4.8945 B 50 10 40 5.485 Theoretical 54.25 25.75 20 composition ratio Analyzed 29.8 14.2 11.2 44.8 value Comparative A 50 50 0 6.025 example 5-8 B 50 0 50 5.35 C 67 33 0 4.8945 B 50 0 50 5.35 Theoretical 54.25 20.75 25 composition ratio Analyzed 29.8 11.4 13.8 45 value

TABLE 16 Ta-containing other lamination structures Me:Ta Thermal expansion coefficient Lamination Composition (at %) (×10⁻⁶/ Division structure Al Ti Ta N ° C.) Comparative A 50 50 0 6.025 example 5-1 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 40 10 5.89 Theoretical 54.25 43.25 2.5 composition ratio Analyzed 29.6 23.8 1.4 45.2 value Comparative A 50 50 0 6.025 Example 5-2 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 25 25 5.6875 Theoretical 54.25 39.5 6.25 0 composition ratio Analyzed 29.9 22 3.4 44.7 value Comparative A 50 50 0 6.025 example 5-3 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 10 40 5.485 Theoretical 54.25 35.75 10 composition ratio Analyzed 29.8 19.7 5.5 45 value Comparative A 50 50 0 6.025 example 5-4 C 67 33 0 4.8945 A 50 50 0 6.025 B 50 0 50 5.35 Theoretical 54.25 33.25 12.5 composition ratio Analyzed 29.8 20.3 7.1 42.8 value

TABLE 17 Cr-containing A-B-C three-layer repetitive lamination structures Me:Cr Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Cr N (×10⁻⁶/° C.) Example 6-1 A 50 50 0 6.025 B 50 40 10 5.44 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 30.8 22.6 1.8 44.8 value Example 6-2 A 50 50 0 6.025 B 40 35 25 5.2275 C 67 33 0 4.8945 Theoretical 52.3 39.3 8.3 composition ratio Analyzed 29.3 21.6 4.6 44.5 value Example 6-3 A 50 50 0 6.025 B 25 35 40 5.3475 C 67 33 0 4.8945 Theoretical 47.3 39.3 13.3 composition ratio Analyzed 26 21.6 8.8 43.5 value Comparative A 50 50 0 6.025 example 6-1 B 15 35 50 5.4275 C 67 33 0 4.8945 Theoretical 44 39.3 16.7 composition ratio Analyzed 24.4 21.6 9.2 44.8 value Comparative A 50 50 0 6.025 example 6-2 B 50 25 25 4.5625 C 67 33 0 4.8945 Theoretical 55.7 36 8.3 composition ratio Analyzed 30.6 19.8 5.6 44 value

TABLE 18 Cr-containing A-B-C-B four-layer repetitive lamination structures Me:Cr Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Cr N (×10⁻⁶/° C.) Example 6-4 A 50 50 0 6.025 B 50 40 10 5.44 C 67 33 0 4.8945 B 50 40 10 5.44 Theoretical 54.25 40.75 5 composition ratio Analyzed 31.4 22.4 2.8 43.4 value Example 6-5 A 50 50 0 6.025 B 40 35 25 5.2275 C 67 33 0 4.8945 B 40 35 25 5.2275 Theoretical 49.25 38.25 12.5 composition ratio Analyzed 27.1 21.4 6.5 45 value Comparative A 50 50 0 6.025 example 6-3 B 25 35 40 5.3475 C 67 33 0 4.8945 B 25 35 40 5.3475 Theoretical 41.75 38.25 20 composition ratio Analyzed 23 21.7 11.2 44.1 value Comparative A 50 50 0 6.025 example 6-4 B 15 35 50 5.4275 C 67 33 0 4.8945 B 15 35 50 5.4275 Theoretical 36.75 38.25 25 composition ratio Analyzed 20.2 21.3 13.8 44.7 value Comparative A 50 50 0 6.025 example 6-5 B 50 25 25 4.5625 C 67 33 0 4.8945 B 50 25 25 4.5625 Theoretical 54.25 33.25 12.5 0 composition ratio Analyzed 28.8 21.6 7.1 42.5 value

TABLE 19 Si-containing A-B-C three-layer repetitive lamination structures Me:Si Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Si N (×10⁻⁶/° C.) Example 7-1 A 50 50 0 6.025 B 50 40 10 5.41 C 67 33 0 4.8945 Theoretical 55.7 41 3.3 composition ratio Analyzed 30.8 22.6 1.7 45.1 value Example 7-2 A 50 50 0 6.025 B 40 35 25 5.1525 C 67 33 0 4.8945 Theoretical 52.3 39.3 8.3 composition ratio Analyzed 28.9 22.1 4.1 44.9 value Example 7-3 A 50 50 0 6.025 B 25 35 40 5.2275 C 67 33 0 4.8945 Theoretical 47.3 39.3 13.3 composition ratio Analyzed 26 23 6.9 44.1 value

TABLE 20 Si-containing A-B-C-B four-layer repetitive lamination structures Me:Si Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti Si N (×10⁻⁶/° C.) Example 7-4 A 50 50 0 6.025 B 50 40 10 5.41 C 67 33 0 4.8945 B 50 40 10 5.41 Theoretical 54.25 40.75 5 composition ratio Analyzed 30.9 22.5 2.3 44.3 value Example 7-5 A 50 50 0 6.025 B 40 35 25 5.1525 C 67 33 0 4.8945 B 40 35 25 5.1525 Theoretical 49.25 38.25 12.5 composition ratio Analyzed 27.1 21 6.7 45.2 value Comparative A 50 50 0 6.025 example 7-1 B 25 35 40 5.2275 C 67 33 0 4.8945 B 25 35 40 5.2275 Theoretical 41.75 38.25 20 composition ratio Analyzed 24.1 21 10.4 44.5 value

TABLE 21 V and Si-containing lamination Structures Me: V + Si Thermal expansion Lamination Composition (at %) coefficient Division structure Al Ti V Si N (×10⁻⁶/° C.) Example A 50 50 0 6.025 8-1 B 50 40 10 0 5.96 C 67 33 0 0 4.8945 D 50 40 0 10 5.41 Theoretical 54.3 40.8 2.5 2.5 composition ratio Analyzed 29.8 22.4 1.4 1.4 45.0 value Example A 50 50 0 6.025 8-2 B 50 35 10 5 5.6525 C 67 33 0 0 4.8945 Theoretical 55.7 39.3 3.3 1.7 composition ratio Analyzed 30.4 21.6 1.8 1.0 45.1 value

Cutting Performance Test Results

With respect to the hard coatings formed as described above, a heat-resistance crack test, a milling wear resistance test, and a milling shock resistance test were performed and the cutting performance was evaluated.

At his point, the used I/S model number was APMT1604PDSR-MM and the used cutter model number was AMC3063HS.

(1) Heat Resistance Evaluation Conditions

Workpiece to be cut: STS316 (100×200×300)

Cutting speed: 120 m/min

Feed per tooth: 0.25 mm/tooth

Depth of cut: 10 mm

Radial depth of cut: 5 mm

Dry-type is applied, and states of tool noses are compared in batch after processing 780 cm.

(2) Milling Wear Resistance Characteristic Evaluation Conditions

Workpiece to be cut: SCM440 (100×200×300)

Cutting speed: 250 m/min

Feed per tooth: 0.1 mm/tooth

Depth of cut: 10 mm

Radial depth of cut: 5 mm

Dry type is applied.

(3) Milling Shock Resistance Characteristic Evaluation Conditions

Workpiece to be cut: SCM440 3-line diaphragm plate (100×30×300)

Cutting speed: 180 m/min

Feed per tooth: 0.15 mm/tooth

Depth of cut: 10 mm

Radial depth of cut: 5 mm

Dry type is applied.

The above cutting performance test results are arranged in tables below.

TABLE 22 Zr-containing thin film test results Cutting performance evaluation results Number *Containing Thin film Thin film of Zr thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.0 38 5 2400 Normal wear 400 Chipping Example 1-1 Comparative 2.9 38 6 2400 Normal wear 400 Chipping Example 1-2 Comparative 3.0 38.5 6 1650 Chipping 400 Chipping Example 1-3 wear Comparative 3.2 37.8 6 2400 Normal wear 400 Chipping Example 1-4 Comparative 3.4 33.1 8 1600 Chipping 310 Chipping Example 5 (common) Comparative 3.1 37.5 5 2100 Normal wear 400 Chipping Example 6 Comparative 3.4 36.2 0 1800 Normal wear 50 Chipping Example 7 Comparative 3.2 36.9 0 1400 Initial 50 Initial Example 8 delamination delamination Example 1-1 3.1 37.5 1 2400 Normal wear 800 Chipping Example 1-2 2.9 37.7 0 2100 Normal wear 800 Chipping Example 1-3 2.8 37.9 0 2100 Normal wear 750 Chipping Example 1-4 2.8 42 0 2400 Normal wear 780 Chipping Example 1-5 2.7 41.5 0 2400 Normal wear 700 Chipping

TABLE 23 Hf-containing thin film test results Cutting performance evaluation results Number *Containing Thin film Thin film of Hf thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.3 36 5 2100 Normal wear 400 Chipping Example 2-1 Comparative 3.3 36.2 4 2100 Normal wear 400 Chipping Example 2-2 Comparative 3.3 36.2 3 2200 Normal wear 400 Chipping Example 2-3 Comparative 3.1 36.2 5 2200 Normal wear 400 Chipping Example 2-4 Comparative 3.2 35.8 0 1980 Normal wear 310 Chipping Example 2-6 (common) Comparative 3.1 35.5 1 250 Initial 400 Chipping Example 2-7 delamination Comparative 2.9 36 0 100 Initial 50 Chipping Example 2-8 delamination Example 2-1 3.0 35.5 1 2200 Normal wear 800 Chipping Example 2-2 2.9 35.8 1 2200 Normal wear 800 Chipping Example 2-3 2.9 36.1 1 2200 Normal wear 750 Chipping Example 2-4 3.0 38 0 2300 Normal wear 780 Chipping Example 2-5 2.9 38.1 0 2300 Normal wear 700 Chipping

TABLE 24 V-containing thin film test results Cutting performance evaluation results Number *Containing Thin film Thin film of V thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.0 36 4 2200 Normal wear 650 Chipping Example 3-1 Comparative 2.9 36.1 6 2200 Normal wear 650 Chipping Example 3-2 Comparative 3.3 36.2 5 2100 Normal wear 450 Chipping Example 3-4 Comparative 3.1 37 6 2100 Normal wear 650 Chipping Example 3-5 Comparative 3.1 32.1 5 2100 Normal wear 600 Chipping Example 3-7 Comparative 3.1 36.2 7 2000 Normal wear 600 Chipping Example 3-8 Comparative 3.1 32 5 1200 Initial 250 Chipping Example 3-11 delamination Comparative 3.1 30.8 5 800 Chipping 480 Chipping Example 3-12 Comparative 3.1 32.5 8 800 Chipping 400 Chipping Example 3-13 Example 3-1 3.0 35.5 0 2200 Normal wear 900 Chipping Example 3-2 3.1 35.5 1 2200 Normal wear 900 Chipping Example 3-3 3.1 35.3 2 2100 Normal wear 800 Chipping Example 3-4 3.0 38 0 2200 Normal wear 900 Chipping Example 3-5 3.1 37.1 0 2200 Normal wear 900 Chipping

TABLE 25 Nb-containing thin film test results Cutting performance evaluation results *Containing Thin film Thin film Number of Nb thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.1 36.8 7 1700 Chipping 200 Example 4-1 Comparative 3.0 36.5 8 1700 Chipping 300 Example 4-2 Comparative 3.0 34 8 1800 Chipping 300 Example 4-3 Comparative 3.0 33.5 7 1800 Chipping 280 Example 4-4 Comparative 3.1 37 8 2000 Chipping 250 Example 4-5 Comparative 2.9 36.1 8 1900 Chipping 250 Example 4-6 Comparative 3.0 33.2 9 1800 Chipping 250 Example 4-7 Comparative 3.2 30.5 8 1800 Chipping 200 Example 4-8

TABLE 26 Ta-containing thin film test results Cutting performance evaluation results Number *Containing Thin film Thin film of Ta thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.2 37 5 2300 Normal 450 Chipping Example 5-1 wear Comparative 3.1 37.1 6 2300 Chipping 450 Chipping Example 5-2 Comparative 3.1 37.2 7 1650 Chipping 450 Chipping Example 5-3 Comparative 3.1 36.8 5 2000 Chipping 400 Chipping Example 5-4 Comparative 3.0 36.1 6 2100 Normal 400 Chipping Example 5-6 wear Comparative 3.2 36.5 5 1250 Excessive 100 Initial Example 5-7 wear delamination Comparative 3.3 36.2 6 1000 Excessive 100 Initial Example 5-8 wear delamination Example 5-1 3.0 37.5 1 2400 Normal wear 800 Chipping Example 5-2 3.0 37.6 0 2100 Normal wear 800 Chipping Example 5-3 3.1 37.8 0 2100 Normal wear 750 Chipping Example 5-4 3.0 41 0 2400 Normal wear 900 Chipping Example 5-5 3.1 37.2 0 2400 Normal wear 800 Chipping

TABLE 27 Cr-containing thin film test results Cutting performance evaluation results Number *Containing Thin film Thin film of Cr thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.1 37.1 0 2100 Normal wear 600 Chipping Example 6-1 Comparative 3.2 37.5 5 2100 Normal wear 600 Chipping Example 6-2 Comparative 3.2 36.1 0 1200 Excessive 250 Chipping Example 6-3 wear Comparative 3.3 35.5 0 800 Excessive 550 Chipping Example 6-4 wear Comparative 3.3 37.1 4 1500 Excessive 500 Chipping Example 6-5 wear Example 6-1 3.1 37 1 2200 Normal wear 900 Chipping Example 6-2 3.0 37.5 0 2200 Normal wear 900 Chipping Example 6-3 3.1 37 1 2100 Normal wear 850 Chipping Example 6-4 2.8 37.8 0 2300 Normal wear 900 Chipping Example 6-5 3.1 37.2 0 2300 Normal wear 900 Chipping

TABLE 28 Si-containing thin film test results Cutting performance evaluation results *Containing Thin film Thin film Number of Si thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Comparative 3.0 48.9 1  280 Chipping  20 Initial Example 7-1 delamination Example 7-1 3.0 40.5 0 2300 Chipping 750 Chipping Example 7-2 3.0 44 2 2400 Chipping 700 Chipping Example 7-3 3.1 45.8 2 2600 Chipping 700 Chipping Example 7-4 3.0 42 0 2500 Chipping 800 Chipping Example 7-5 2.8 44.9 0 2600 Chipping 700 Chipping

TABLE 29 V-and Si-containing thin film test results Cutting performance evaluation results Number *Containing Thin film Thin film of V + Si thickness hardiness thermal Milling wear Milling shock Division (μm) (GPa) cracks resistance (cm) resistance (cm) Example 8-1 3.1 41.5 0 2500 Normal wear 800 Chipping Example 8-2 3.1 41 0 2500 Normal wear 850 Chipping

As shown in Table 22, in the case of comparative example 5 which does not include an AlTiZrN layer, it can be found that not only the hardness of a thin film is low, but also the number of thermal cracks is eight which is more than those in other examples or comparative examples. As a result, it may be found that physical properties are relatively worse than other hard coatings particularly in the aspect of milling wear resistance.

In addition, it may be found that although comparative examples 1-1 to 1-4, and 1-6 to 1-8 include AlTiZrN layers, the number of thermal cracks is great, milling shock resistance is extremely low, and thus, overall physical properties of the hard coatings are lower than examples 1-1 to 1-5 of the present invention.

Similarly, as shown in Table 23, in the case of examples 2-1 to 2-5 of the present invention comparative example 5 which includes an AlTiHfN layer, it can be found that the number of thermal cracks is smaller than comparative examples 2-1 to 2-4 and 2-6 to 2-8, or overall physical properties including milling wear resistance and milling shock resistance are remarkably improved.

This tendency similarly appears also in hard coatings containing V, Ta, Cr, Si, V or Si.

Meanwhile, as illustrated in Table 25, it may be found that when an AlTiNbN layer, containing Nb which does not have a thermal expansion coefficient between those of TiAlN and AlTiN, is used, not only the great number of thermal cracks but also both low milling wear resistance and low milling shock resistance are exhibited.

In addition, according to examples of the present invention, it may be found that hard coatings including an AlTiMeN layer including Zr, Ta, Si, V or Si exhibit higher physical properties, and thus, these components may be more favorably used.

That is, as confirmed through the examples and comparative examples, hard coatings according to the present invention may maintain high milling wear resistance and milling shock resistance while remarkably reducing thermal cracks, and thus may contribute to improve the service life of cutting tools. 

The invention claimed is:
 1. A hard coating formed on the surface of a base material for a cutting tool by a PVD method, the hard coating comprising a thin film layer which has a total thickness of 0.5-10 μm and has an overall composition of Al_(1-a-b)Ti_(a)Me_(b)N (0.2<a≤0.6, 0<b≤0.15), wherein the Me is a nitride constituent element having a thermal expansion coefficient of greater than 2.7×10⁻⁶/° C. and less than 9.35×10⁻⁶/° C., wherein the thin film layer has a nano-multilayered-structure of thin layers A, B and C, a thin layer B being disposed between a thin layer A and a thin layer C as a A-B-C layer or a C-B-A layer, the A-B-C layer or the C-B-A layer being repeatedly laminated at least once, wherein, the thin layer A has a composition of Ti_(1-a)Al_(a)N (0.3≤a<0.7), the thin layer B has a composition of Ti_(1-y-z)Al_(y)Me_(z)N (0.3≤y<0.7, 0.01≤z<0.5), and the thin layer C has a composition of Al_(1-x)Ti_(x)N (0.3≤x<0.7), wherein kA is the thermal expansion coefficient of the thin layer A, kB is the thermal expansion coefficient of the thin layer B, and kC is the thermal expansion coefficient of the thin layer C, the thin film layer satisfying the relationship of kA>kB>kC, wherein the difference in the thermal expansion coefficients of Me nitrides and the thermal expansion coefficients of the AlN and TiN is at least 1.0×10⁻⁶/° C.
 2. The hard coating of claim 1, wherein the Me comprises one or more selected from among Si and Groups 4a, 5a, and 6a elements.
 3. The hard coating of claim 1, wherein the Me comprises one or more selected from among Si, Zr, Hf, V, Ta, and Cr. 